1. Field of the Invention
The present invention relates to a nonvolatile memory device and a semiconductor device including the memory device.
2. Description of the Related Art
In a semiconductor device, a volatile memory such as a static random access memory (SRAM) is used for a memory portion where data is stored when high speed operation is needed. On the other hand, when data needs to be held while power is not supplied, a nonvolatile memory such as an electrically erasable programmable read-only memory (EEPROM) or a flash memory is used.
As a method for holding data certainly while power is not supplied, there is a backup method in which a volatile memory capable of high speed operation, such as an SRAM, is used while power is supplied to the semiconductor device and immediately before supplying power is stopped, data is written to a nonvolatile memory which operates at low speed.
As an SRAM which is non-volatized (also called nonvolatile SRAM) so as to store data even when the power supply of the SRAM which is a volatile memory is turned off, a nonvolatile memory in which a ferroelectric capacitor is provided in a memory cell has been actively researched and developed (see Patent Document 1).